1. Field of the Invention
The present invention relates to a manufacturing method of a polymer film usable as an optical functional film such as a protective film of a polarizing plate of a liquid crystal display, an optical compensation film or the like.
2. Description of the Related Arts
A polymer film (hereinafter called as the film) has superior optical transparency and flexibility, and allows reduction in thickness and weight thereof. Accordingly, the polymer film is used as various optical functional films. Especially, a cellulose ester film such as a cellulose acylate film is used as a photosensitive film, a protective film of a polarizing plate of a liquid crystal display (LCD) whose market is expanding recently, an optical compensation film and the like, because of having toughness and low birefringence.
As methods for producing the film, there are a melt extrusion method and a solution casting method. The melt extrusion method has an advantage in high productivity and low cost for a production apparatus, because polymers are melted and then extruded from an extruder to produce the film. However, in this method, accuracy of the film thickness is low, and minute streaks (die lines) are generated on the film. Accordingly, it is difficult to produce the film with high quality sufficient to be used as the optical film. In contrast, in the solution casting method, polymers are dissolved to a solvent to prepare a polymer solution (dope), and the dope is cast on a moving support to form a casting film. After having a self-supporting property, the casting film is peeled from the support as a wet film, and dried so as to become a film. The film produced in this method is more excellent in optical isotropy and uniformity of thickness and has less foreign particles than that obtained in the melt extrusion method. For above reasons, most of the optical films for the LCD and the like are produced by the solution casting method.
The LCDs are classified into various modes such as VA (Vertical Aligned) mode for high-contrast, IPS (In-Plane Switching) mode for wide viewing angle, and OCB (Optically Compensated Birefringence) mode for wide viewing angle and quick response, based on an operating mode of a liquid crystal layer. The optical film for the LCD is required that a phase difference (retardation) according to degree (strength) of orientation of the film is large (high retardation value) to realize high contrast and wide view angle, regardless of the mode of LCD.
To give the high retardation value to the film, generally tension is applied on the film in the width direction or the longitudinal direction thereof to stretch it so that molecular orientation in the film is controlled. Recently proposed in the above method is that a content of the remaining solvent and other conditions in the film at the stretching are determined in certain ranges, to give much higher retardation value to the film (for example, Japanese Patent Laid-Open Publications No. 2003-170492, No. 2004-314529 and No. 2005-173024). In addition, there is a proposed method in which preparing and casting method of the dope, manufacturing conditions of the film and so on are determined in detail, so as to manufacture the film having superior optical properties such as high retardation value and superior transparency (for example, Japanese Patent Laid-Open Publication No. 2005-104148).
Although these methods can produce the film having high retardation value in certain level, the required value for the retardation is increasing year by year. It is difficult for these methods to satisfy the increased requirement. In addition, when the film is stretched by strong force to increase the retardation value, minute voids are possibly generated in film regions on which strong stresses are applied, that makes the film cloudy (degrading the transparency). To avoid these problems, there is a proposed method to produce the film with high transparency, in which a type of support for casting dope and manufacturing conditions until a casting film being peeled from the support are determined (for example, Japanese Patent Laid-Open Publication No. 2003-071863).
According to the method of the Japanese Patent Laid-Open Publication No. 2003-071863, the film having the superior transparency can be obtained by procedures that a casting belt wound around a drum is used as the support, the drum is cooled to form the casting film in low temperature, and time taken for peeling the casting film from the casting belt is determined. However, there is no statement in this publication about how to realize the required high retardation value for the film to be used in the LCD and the like. In addition, when the cooled drum as the above method is used, the casting film is formed by cooling solidification of the dope. Since the wet film right after the peeling of the casting film from the support is considerably soft, it is difficult to dry the wet film while convey it stably.